Conventional transfer molding systems employed for integrated circuit packaging generally utilize hardened steel mold surfaces to clamp onto the lead frame or packaging substrate. When the integrated circuit die being packaged requires an exposed active area after packaging (e.g., for capacitive or light sensors therein) as illustrated in FIG. 3, the mold system 300 typically includes lower and upper mold portions 301 and 302, respectively, receiving an integrated circuit die 303 mounted on a packaging substrate 304. Upper mold 302 defines a cavity 305 around integrated circuit die 303 and a portion of packaging substrate 304, with a portion of the upper mold 302 contacting the active area 306 to prevent injected encapsulating material from leaking onto that surface, leaving the active area 306 exposed after packaging.
As with other integrated circuits, sensor integrated circuit die are commonly affixed to rigid lead frames or packaging substrates by an adhesive paste. Due to the nature of the materials and the process, upper surface height variations of the integrated circuit die with respect to the lead frame or packaging substrate are inevitable. Die attach glue thickness non-uniformity from one integrated circuit package to another, or non-planarity of the adhesive for a particular integrated circuit die (exaggerated in FIG. 3), can cause imperfect contact of the upper mold with the active area, resulting in leakage of the encapsulating material onto the active area or damage to the integrated circuit die active area.
At molding, compensation for the height variations is necessary, and is typically achieved in one of several ways: compliant top mold; compliant bottom mold; or complaint material coating the clamping contact area of the integrated circuit die. However, each of these techniques is unsatisfactory in some aspect.
There is, therefore, a need in the art for alternative methods of transfer molding during packaging of exposed active area integrated circuits that either tolerates sensor surface height variations or minimizes the sensor surface height variations.